The present invention relates to an apparatus for adjusting automatically a microscope in focus with a high accuracy.
In steps of manufacturing a semiconductor integrated circuit there is a step for photoetching a silicon wafer of high purity. In this manufacturing process on a surface of the silicon wafer having deposited a photo resist thereon is placed a mask having a given pattern, the photo resist is irradiated by visible light or ultraviolet ray through said mask and then the silicon wafer is selectively etched after developing the photo resist. Such a mask is generally formed by vapor depositing a uniform metal film such as chromium and nickel on a glass plate and the metal layer is selectively photoetched in accordance with the given pattern. In this case before forming the pattern in the metal film it is necessary to check and assure that the metal film does not include any serious pin hole. In general when the metal film includes a pin hole having a diameter larger than 2.mu., integrated circuits manufactured using the mask which has been formed by such a metal film might be inferior. Particularly in integrated circuits of high integration the metal film having a pin hole of a diameter of about 1.mu. results in unfavorable semiconductor integrated circuits.
In the invention a microscope is utilized for checking pin holes in the above mentioned metal film. In order to find a pin hole having a diameter larger than 1.mu., it is necessary to use a microscope which has a magnification of about 200. The microscope having such a large magnification has a short focal depth of 4 to 5.mu.. On the contrary the surface of the glass plate of the mask is ground and polished to only such an extent that its surface fluctuation amounts to about 10.mu.. Thus during the examination of the metal film by relatively moving the microscope and glass plate in a direction parallel to the surface of the glass plate, there is a fear that the microscope may become out of focus. Since the metal film deposited on the glass plate has been carefully treated and thus there are only a few pin holes in the metal film, it is difficult for an observer to be aware during the examination that the microscope becomes out of focus and thus the observer fails to find pin holes. Moreover even if the observer finds that the microscope is out of focus, it is quite difficult to readjust accurately the microscope into focus when there is no pin hole within the field of view. In practice when the microscope is to be readjusted, the microscope and the glass plate placed on a stage of the microscope are moved relative to each other so that an edge portion of the glass plate comes in the field of view. Therefore readjustment of the microscope is quite troublesome.
Further in order to examine the glass plate having an area of, for example 50mm.times.50mm using the microscope it takes several hours. During a such long time period the observer must be always paying attention whether or not the microscope is in focus, which is extremely annoying and the observer is very much tired. Moreover in order to design an apparatus for automatically checking pin holes in the metal film, the above mentioned defocus of the microscope will be a serious drawback and an accurate examination could not be expected. Therefore it is required to develop an auto-focusing apparatus of high accuracy. Many auto-focusing apparatuses have been proposed, but their accuracy is very low. Therefore the known auto-focusing apparatus could not be applied to the microcscope having a relatively short focal depth.